We have studied the arousal effect of suxamethonium on the auditory evoked response (AER) of the electroencephalogram (EEG) in 40 ASA I and II patients during isoflurane anaesthesia. After induction of anaesthesia, the patient's lungs were ventilated for 20 min with 0.6 MAC end-expiratory isoflurane (0.59-0.77% depending on the age of the patient), and 50% nitrous oxide in oxygen. The patients were then allocated randomly to one of two groups: 21 received suxamethonium 1 mg kg-1, while 19 were given saline. The AER before and after administration of suxamethonium or saline was compared to determine the changes in Pa and Nb amplitudes and latencies. Pa amplitude after suxamethonium increased by 53% (95% confidence interval (CI) 15, 104%) compared with a reduction in Pa amplitude in the saline group of 19% (95% CI, -41, 12%) (P = 0.004) suggesting an arousal effect. Similarly, Nb amplitude increased in the suxamethonium group by 47% (95% CI, 3, 110%) and decreased in the saline group by 11% (95% CI, -33, 19%) (P = 0.03). We conclude that suxamethonium caused arousal according to the AER and postulate that this may have been caused by increased muscle afferent activity after stimulation of muscle spindles, although further studies are required to confirm this.